Wilder had to survive a beating from Ortiz before landing a brutal punch in the 10th round which knocked out the Cuban boxer.
Ortiz started better of the two fighters, sending Wilder to the canvas twice before the 2: 05-minute mark.
Wilder recovered to win the fifth round, attacking Ortiz with the speed and vigour he is known for but failed to finish the job in the sixth.
In the seventh, Ortiz attacked, this time getting Wilder with some punches.
It was all going Ortiz’s way and he should have done more to get his opponent down before the 10th-round when he was knocked out by a brutal punch.
Wilder had been looking for that moment, and when it came, it was perfect, the punch landing on Ortiz and sending him to the floor.
Then the fight was won.
Wilder is ready for Joshua
Wilder has been taunting Anthony Joshua for almost a year now and after the fight, he continued.
However, Joshua has to beat his next opponent on Saturday, March 31 for the WBO belt.
If he comes out on top, a much-anticipated fight against Wilder should be ready and the American is battle ready.
“I'm ready right now, I always said that I want to unify. I'm ready whenever those guys are,” Wilder said after the win against Ortiz.
“I am the baddest man on the planet and I proved that tonight. This solidified my position at the top of the food chain tonight.
“Soon there will be one champion, one face, one name – that will be Deontay Wilder. It's not about how big your muscles are – it's about the heart.
“I am looking forward to Cardiff, I have never been there. I have a lot of friends in the UK. I am going to enjoy myself and look forward to fighting the winner – if they are up for the test.”